arm64: dts: renesas: r8a77961: Add INTC-EX device node

Populate the device node for the Interrupt Controller for External
Devices (INTC-EX) on R-Car M3-W+, which serves external IRQ pins
IRQ[0-5].

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/6f3cc1a1b6d777e743a7a9e66a80aaf9d5232a4f.1622547125.git.geert+renesas@glider.be